In broadband wireless telecommunications, orthogonal frequency division multiplexing (OFDM) is a special case of multicarrier modulation (MCM), which is the principle of transmitting data by dividing the stream into several parallel bit streams and modulating each of these data streams onto individual carriers or subcarriers. Conventional OFDM systems utilize the Inverse Fast Fourier Transform (IFFT) and Fast Fourier Transform (FFT) to modulate and demodulate, respectively, information data.
OFDM systems are, however, susceptible to frequency offsets, which may result in a loss of orthogonality in the subcarriers and, as such, in Inter Carrier Interference (ICI). Such frequency offsets may result from a number of possible causes, including differences in the frequency of a transmitter and receiver due to local oscillator tolerance; Doppler shift due to the motion of mobile station and reflecting objects through a propagation path; and additive noise which may add instantaneous phase noise.
As OFDM transmission is heavily disturbed by the presence of a frequency offset, it is necessary to precisely correct the offset at the receiver. Many solutions to this problem are present in the literature and prior art, with processing that can be both in the time domain (pre-FFT) or in the frequency domain (post-FFT). In particular, the approach by Moose (“A Technique for Orthogonal Frequency Division Multiplexing Frequency Offset Correction”, IEEE Transactions on Communications, Volume 42, Issue 10, Oct. 1994, Page(s): 2908-2914), proposes a post-FFT algorithm that is based on the repetition of a given training symbol. Such post-FFT algorithm can correct up to ±0.5 intercarrier spacings (also known as “subcarrier spacings”) and needs at least two training symbols; however, the performance of the post-FFT algorithm is equivalent to pre-FFT algorithms based on cyclic prefix (CP) correlation, which can correct up to a maximum of ±0.5 intercarrier spacings. Typically, algorithms proposed in the prior-art for performing fine frequency offset correction are combined with a coarse frequency offset algorithm if the transmission system is specified for operation with a frequency offset larger than ±0.5 intercarrier spacings.
A study of the prior art shows that post-FFT processing is often used for detection of large frequency offsets, but is not often employed in fine frequency synchronization due to the presence of ICI.
Accordingly, a continuing search has been directed to the development of methods which may be utilized to effectuating post-FFT correction of fine frequency offsets.